In machines of the above-mentioned kind, the cable drum is usually rotated by a drive attached to the support and provided with a plug that is inserted into a hole in the flange of the drum. The hole is disposed concentrically relative to the center hole of the drum. These drives usually must be especially made for different drum types and drum sizes, and therefore must be exchanged or adjusted manually when the drum size is changed. Furthermore, it often is difficult to guide the driver plug into the designated hole in the drum. In general, there is some amount of play between the driver plug and the drum hole, and there is a risk of the plug being inserted between spokes, which may be present, rather than in the hole intended for this purpose. This may cause the cable to jolt and thus cause damage to the cable and driver, which can destroy it when the machine is started. The impact stresses thus propagate from the driver to the power unit that rotates the driver. As a result, the drive unit is subjected to undesirable stresses. Because of its asymmetrical construction, the driver also causes an imbalance that subjects the bearings carrying the weight of the drum to excessive stresses. This imbalance is difficult to control. Furthermore, the driver requires a comparatively large motive force in order to attain a specifically defined tensile force in the cable due to the leverage conditions that are determined by the ratio of the length of the driver to the size of the drum. When the drum size is changed a relatively expensive gear box is required in order to adapt the number of revolutions to the size of the drum.
A solution to the problem has been attempted by driving the flange by means of a wheel resting against the flange and driven by means of a driving motor. This eliminates many of the problems encountered when using a driver plug and allows the design of an altogether smaller drive, since the need for installing a bulky motor with gear box together with the bearing bosses is eliminated. This allows production lines to be located somewhat closer to each other, which also means smaller storage halls are needed. This can be of great importance economically.
When driving the flange, the number of revolutions of the drum increases for a smaller drum, allowing a fixed exchange ratio between the motor and the drive wheel, and thereby eliminating the need to exchange the location of the gears when the drum size is changed. The drive wheel in these earlier designs moved against the drum flange by means of an adjustment screw, a hydraulic cylinder or pneumatic cylinder. All these designs are relatively costly and complicated and have not therefore been used extensively. For automatic adjustment relative to the flange of the drum when exchanging the drum, particularly when the drum size is changed, relatively complex sensing and guiding devices are required, primarily with a view to obtaining a relatively precise contact pressure between the drum and the drive wheel. In designs using pneumatic cylinders this problem is avoided to some extent since the contact pressure can be readily controlled by means of the air pressure in the cylinder. However, a problem arises due to the relatively short action range of pneumatic cylinders. In some cases another problem is caused by the tangential force between the drive wheel which counteracts the contact pressure from the cylinder and cancels it at least partially since the wheel is not mechanically locked, but yields to the compressed air.